A "telecine" is an apparatus utilized to transfer a motion picture film, for example a negative film shot in a motion picture camera, to a video signal format for storage or broadcast. Telecines are employed by motion picture and television production and post-production facilities to create video tapes of movies, commercials, news clips, and the like.
In a typical telecine, a source of light such as a flying spot scanner or a laser beam scans the film, or a light bulb projects the image onto a light detecting array, frame by frame, in a line-by-line or raster scan fashion. The light from the light source is transmitted through the film in a "film gate" on the telecine, and received by a light detector positioned to receive light transmitted through the film. A film gate includes an aperture ("frame aperture") for passing light from the light source to the telecine light detector and supports the film as it is scanned. The transmitted light is then converted into electrical signals, converted into an appropriate video signal format such as NTSC or PAL, and stored on a video tape or other storage device.
A flying spot telecine is an apparatus in which consecutive frames in a cinema or movie film are optically scanned in a raster fashion by a light beam of small cross-section (called a "flying spot"), wherein the picture information is thereby modulated on the light beam and converted by one or more opto-electronic detectors into electrical video signals. From the electrical video signals, a television signal is ultimately derived from scanning respective consecutive film frames.
There are two different types of telecines--(1) conventional intermittent or "start/stop", and (2) real-time or continuous motion. In the intermittent or "start/stop" type telecine, each frame of the film is located or "registered" at the frame aperture and held motionless during the scanning process. In the real-time or continuous motion telecine, the film is moved continuously with a film drive to move the film through the machine during the scanning process.
A particular difficulty encountered in telecine devices is the maintenance of the image on the film in reference to a stable image reference. The problem is particularly pronounced when images from different sources are superimposed, for example, when a computer-generated title or other graphics are superimposed against a scene generated by a motion picture film. If the image on the film is not stable relative to the superimposed graphics such as a title, the title may be seen to weave or jitter in relationship to the background. The jitter or weave problem derives from the fact that the positional reference for the film and for the overlaid graphics are not the same. When a film is originally shot in a camera, the image is mechanically registered on a particular film frame relative to the sprocket holes in the film. The tines of a sprocket in the camera engage the sprocket holes in the film to register the film relative to the camera's optics. Such devices, including cameras, are considered mechanically or machine pin registered.
When a film from a motion picture camera is then to be transferred to video in a telecine, the film may not necessarily be machine pin registered but drawn through the machine by a continuous motion film drive. Typically, a drive capstan pulls the film through the film gate. In some continuous motion telecines, the film is edge-guided during the transfer. Film weave or jitter may be introduced as the edge of the film varies relative to the position of the sprocket hole. It is known that film edges tend to vary in distance from the sprocket holes due to manufacturing imperfections and tolerances in the film.
Various approaches have been employed to minimize film weave in film-to-tape transfers. One approach involves use of a single mechanical sprocket drive for moving the film through the telecine. This method introduces small but pronounced speed changes as the sprocket tines enter the sprocket holes, and therefore introduces another source of jitter. Moreover, mechanical sprocket drives are not favored because of the wear and tear on the film.
Other mechanical solutions involve mechanical pin registration with the start/stop devices discussed above. In this approach, the film sprocket holes are lowered, frame by frame, onto stationary register pins with a stepping motor controlled by an auxiliary computer. The film is held steady by pins during the transfer, and then a gate attachment is released to allow the film to advance. Such devices are complicated mechanically, limited in speed, and also produce significant wear and tear on the film because of repeated acceleration and deceleration of the film.
Telecines with continuous film motion are often believed to provide the simplest and best operation if the problems of registration, weave, and jitter can be solved. Flying spot telecines of the continuous motion type require a means for supporting the moving film in a precise position with respect to the optical system of the telecine that focuses an image of the flying spot generated by a cathode ray tube (CRT) through the film. This precise position must be accurately maintained over an area at least as large as one film frame in order to maintain focus. The means for maintaining the position of the film is conventionally referred to as a "film gate". U.K. Patent Specification 1 519 398 describes a film gate that is curved in the direction of motion of the film in an optical system for focusing light on the film in the film gate, together with corrective optics.
One particular advantageous approach to the problem of film weave and jitter is shown in U.S. Pat. No. 5,266,979, entitled "Film Gate for Continuous Motion Telecine", which is owned by the same assignee as the present invention. This patent describes an improved film gate for a continuous motion telecine that employs a precision milled entry guide roller that receives film from a feed spool, edge-stabilizes the film, and directs the film to a precision milled first entry sprocket. The entry sprocket is positioned prior to the frame aperture, pin registers the film, and directs the film across the frame aperture. A precision milled exit sprocket positioned subsequent to the frame aperture receives the film after it passes across the frame aperture and pin registers the film. A precision milled exit guide roller receives the film from the exit sprocket, edge stabilizes the film, and guides the film to a drive capstan and take up reel. The precision sprockets and rollers serve as film stabilizing means. Film jitter and weave are mechanically removed to less than about 20 nanoseconds without the use of complex expensive electronics.
One particular problem with the use of this device is the need for composite work involving a single frame of film. In this type of editing work, the telecine operator desires to overlay or "composite" video, images, or other graphics from another source with the image from the telecine. The film frame serving as a part of the composition must be held stationary during the scan; sometimes the operator must go back and forth between frames and return to a particular frame.
However, with a conventional servomechanism-driven telecine, the operator has no assurance that the servomechanism will return a selected frame to a precise point--the selected frame can be in a slightly different position by several microns and thereby introduces an error. Many telecine servomechanisms can only return a given frame to within about 1/4 of a video scan line, which is insufficient for satisfactory compositing work.
Telecine operators desire a system wherein they can locate a frame (or group of frames) in one roll of film, create a video of a number of frames, run another roll of film to locate another frame or group of frames, and composite the two video images electronically. With a continuous motion telecine, prior to the present invention this has not been possible to do with the desired level of precision.
Accordingly, there is a need for an apparatus that can correct film weave and jitter with a continuous motion telecine, which can also repeatably and reliably pin register a film frame for a stationary frame scan.